In a result that exceeded analysts’ expectations, Syriza and its 40-year-old leader Alexis Tsipras won 149 seats in the 300-seat Greek parliament, just two short of an absolute majority, with most of the votes counted.
The New Democracy party of Prime Minister Antonis Samaras, the former party of government, was routed and reduced to around 76 seats.
Syriza will become the first anti-austerity party to take power in Europe and Tsipras will be Greece’s youngest prime minister in 150 years.
He told thousands of flag-waving supporters in Athens: “Greece is leaving behind disastrous austerity.”
Tsipras repeated his vow to renegotiate the terms of Greece’s 240-billion-euro ($269 billion) bailout with the European Union and the International Monetary Fund, but struck a more conciliatory tone than during his fiery campaign.
“The new Greek government will be ready to cooperate and negotiate for the first time with our peers a just, mutually beneficial and viable solution,” the former Communist youth activist said.
An EU source conceded Sunday that an improved deal on the bailout would have to be struck with the new force in Greek politics.
“We will not escape a re-negotiation,” the source told AFP.
Eurozone finance ministers will have a first chance to discuss the result when they meet Monday.
Tsipras will quickly come under pressure at home to keep pledges such as raising the minimum wage and increasing pensions for the poorest.
The possibility of Greece defaulting on its debt repayments under a Syriza government is likely to spark renewed fears the country could be forced out of the eurozone — a so-called “Grexit”.
In the first market reaction to the vote, the euro briefly sank to a new 11-year low against the dollar of $1.1088 in early Asian trade.
German Chancellor Angela Merkel, seen as the driving force in the EU’s austerity drive, said Friday she wanted Greece to stay in the eurozone “despite the difficulties”.
Syriza says it does not want to give up the euro.
Sunday’s parliamentary polls were Greece’s fourth in five years, including back-to-back votes in 2012.
During that time the economy has shrunk by a quarter and unemployment has soared beyond 25 percent.
Samaras had pleaded with voters not to turn away from the painful public sector spending cuts he had supported, saying they were helping Greece emerge from recession.
But in a sombre statement conceding defeat, he said: “I hand over a country that is part of the EU and the euro. For the good of this country, I hope the next government will maintain what has been achieved.”
Syriza’s victory could inspire other radical parties opposed to austerity in Europe, such as Spain’s Podemos.
Podemos leader Pablo Iglesias told a rally of his supporters in the Spanish city of Valencia on Sunday: “Hope is coming, fear is fleeing. Syriza, Podemos, we will win.”
British Prime Minister David Cameron, the first EU leader to react to Syriza’s victory, tweeted that it would “increase economic uncertainty across Europe”.
However other European countries struck a warmer tone, saying they were prepared to work with a Syriza government.
French President Francois Hollande congratulated Tsipras on his party’s victory and “expressed his desire… to continue the close cooperation between our two countries,” the presidency said.
Italy’s EU affairs minister Sandro Gozi said: “After this vote we will have new opportunities to pursue change in Europe to create growth and investment and fight against unemployment.”
Syriza will seek a coalition partner from either the pro-European To Potami (The River) party, which polled six percent, or the nationalist Independent Greeks.
The neo-Nazi Golden Dawn party performed strongly despite a criminal investigation that has seen many of its top leadership awaiting trial on charges including murder. It will have around 17 seats in the new parliament.
Delighted Syriza supporters gathered around the main campaign platform in a central Athens square to hear Tsipras’ victory speech.
Antonis Balousis, a 54-year-old butcher, said: “This is a very important victory for Greece and Europe.
“We are going to prove that a different kind of politics is possible in Europe.”
Tsipras has said he will immediately seek a 50-percent reduction in Greece’s debts that have spiralled to 318 billion euros.
He says Greece’s creditors have put the country in an “unsustainable” position, forcing it to make ever greater debt repayments while its economic output shrivels.
The IMF has warned Greece that failure to repay its debts will carry “consequences”.
Analysts said the eurozone was set for fresh turbulence.
“A period of uncertainty and heightened market nervousness now seems likely,” said Jonathan Loynes of Capital Economics.
Unicredit chief economist Erik Nielsen said Greece was in for a “volatile month”.
The election was triggered when the Greek parliament failed to elect a new president in December.
Tsipras grew up in a middle-class Athens family and trained as a civil engineer.
An admirer of Che Guevara, the father-of-two has tweaked his image as power beckoned but still steadfastly refuses to wear a tie, in keeping with his radical roots. (AFP)